The invention concerns glass-ceramic precursor glasses, a conversion process from articles made of such glasses into articles consisting of a glass-ceramic having a very low, or even zero thermal expansion, and glass-ceramic articles obtained from such glasses, especially cooktop plates.
Glass-ceramic articles have been known for about 30 years, since their invention in U.S. Pat. No. 2,920,971 (Stookey). Even though the applications for glass-ceramics are very diverse, they are particularly useful for the fabrication of cookware. Glass-ceramics are very much appreciated by the customers in part for their aesthetic appearance, but also for their physical and chemical properties. For instance, not only can their visual aspect be varied almost infinitely, but, in addition, their composition can be adjusted to make them highly resistant to acids and alkali, as well as to confer upon them a high mechanical resistance, frequently two-to-three times higher than that of glass articles of identical shape. Such a combination of properties has allowed their use as cooktop plates, in addition to their use as cookware that can be used up to much higher temperatures than classical borosilicate glasses.
It is well known that the production of glass-ceramic articles requires three major steps: melting a mixture of raw materials usually containing a nucleating agent; forming and cooling the glass below its transformation range; and crystallizing the glass article ("ceramming") by an appropriate thermal treatment. In general, this last part of the process is performed in two steps: the glass article is first heated up to a temperature slightly higher than the transformation range in order to generate the nuclei. The temperature is then increased so that crystal growth can occur on the nuclei.
On the one hand, crystalline growth being enhanced by a temperature increase, crystallization of the main crystal phase is usually performed at as high a temperature as possible to minimize the duration of the thermal treatment, and thereby the production costs. On the other hand, the article has the tendency to deform during ceramming due to thermal heterogeneities. This deformation is indeed proportional to the size of the ware, and may therefore be large in the case of cooktop plates. It is then necessary to find a compromise between optimizing the crystal growth rate and minimizing the distortion of the article. In all cases, glass composition must be adjusted so that it ensures rapid crystalline growth.
When glass-ceramic plates or sheets must be fabricated, the glass composition must also guarantee a glass viscosity compatible with the usual rolling techniques. The plates or sheets then undergo a ceram treatment such as described above.
For an application as cooktop plates, the transmission of the glass-ceramics in the visible range must be low enough to prevent dazzling of the user by the underlying heating elements (especially true when halogen lamps are used), but also high enough so that, for safety reasons, the heating element can be detected during its use by the eye. In addition, the infrared transmission must be high to optimize the efficiency of heating and cooking.
Finally, the thermal expansion coefficient of the glass-ceramic must preferably be very low to prevent breakage by thermal shock.
The present invention describes glasses having the following characteristics: (a) a viscosity required for the rolling of plates or sheets; (b) the ability to be cerammed into transparent glass-ceramics, the transmissions of which in the visible and infrared ranges can be adjusted; (c) a very low or zero thermal expansion coefficient; and (d) a minimal distortion during a very short ceram cycle (thermal crystallization treatment).
The invention also gives a process to transform a glass article of the invention into a glass-ceramic article by a thermal treatment of short duration.
Another aspect of the invention is to provide glass-ceramic articles having high transmissions in the visible and infrared ranges and a very low or zero thermal expansion coefficient, in particular heat-resistant plates (for instance, woodstove windows or fire protection windows) or cookware.
The invention also concerns articles made of a transparent glass-ceramic, presenting a low visible transmission and a high infrared transmission, and a very low or zero thermal expansion coefficient, in particular cooktop plates.